Global Change Biology (2004) 11, 356–367, doi: 10.1111/j.1365-2486.2004.00882.x
Patterns of nitrogen and sulfur accumulation and
retention in ombrotrophic bogs, eastern Canada
T I M M O O R E *w , C H R I S T I A N B L O D A U z, J U K K A T U R U N E N *w , N I G E L R O U L E T *w and
PIERRE J. H. RICHARDw§
*Department of Geography, McGill University, 805 Sherbrooke Street West, Montréal, QC, Canada H3A 2K6, wCentre for Climate
& Global Change Research, McGill University, 805 Sherbrooke Street West, Montréal, QC, Canada H3A 2K6, zLimnological
Research Station, Department of Hydrology, University of Bayreuth, 95444 Bayreuth, Germany, §Département de Géographie,
Université de Montréal, C.P. 6128 Centre-ville, Montréal, QC, Canada H3C 3J7
Abstract
Nitrogen (N) and sulfur (S) play important roles in peatlands, through their influence on
plant production and peat decomposition rates and on redox reactions, respectively, and
peatlands contain substantial stores of these two elements. Using peat N and S
concentrations and dry bulk density and 210Pb dating, we determined the rates of N and
S accumulation over the past 150 years in hummock and hollow profiles from 23
ombrotrophic bogs in eastern Canada. Concentrations of N and S averaged 0.80% and
0.18%, respectively, generally increased with depth in the profile and there was a weak
but significant correlation between N and S concentrations. Rates of N and S
accumulation over the past 50–150 years ranged from 0.5 to 4.8 g N m2 yr1 and from
0.1 to 0.9 g S m2 yr1. There were significant but weak correlations between C, N and S
accumulation rates over 50-, 100- and 150-year periods. Over the last 50 years, rates of S
accumulation showed little differentiation between hummocks and hollows, whereas
the pattern for N accumulation was more variable (hummock minus hollow rate ranged
from 1 to 1 1.5 g N m2 yr1), with hummocks generally having a larger N accumulation rate, correlated with the rate of carbon (C) accumulation. There was a modest but
significant positive correlation between 50-year rates of N accumulation and wet
atmospheric deposition of N measured between 1990 and 1996, with accumulation rates
about four times that of wet deposition. The difference between deposition and
accumulation of N is attributed to organic N deposition, dry deposition and N2 fixation.
A weaker, but still significant, correlation was observed between 50-year S accumulation
and 1990–1996 wet atmospheric S deposition, with about 75% of the deposited S
accumulating in the peat. A laboratory experiment with peat cores exposed to varying
water table position and simulated N and S deposition, showed that on average 87% and
98% of the deposited NH41 and NO
3 , respectively, and 58% of the deposited S were
retained in the vegetation and unsaturated zone of the cores, supporting the results from
the field study.
Keywords: atmospheric deposition, bog, carbon, nitrogen, peat, sulfur
Received 9 April 2004; received in revised form 16 August 2004; accepted 6 September 2004
Introduction
The role of northern peatlands as major global sinks of
carbon (C) has been well established: they have
Correspondence: Tim Moore, Department of Geography, McGill
University, 805 Sherbrooke Street West, Montréal, QC, Canada
H3A 2K6. tel. 1 514 398 4961, fax 1 514 398 7437,
e-mail: [email protected]
356
accumulation rates of between 10 and 30 g C m2 yr1,
and contain between 5 and 250 kg C m2 with a global C
mass of between 250 and 450 Pg (Gorham, 1991;
Turunen et al., 2001). The accumulation and retention
of nitrogen (N) and sulfur (S) in peatlands are less
established. N is an important, growth-limiting nutrient
in many peatlands (e.g. Aerts et al., 1992; Berendse et al.,
2001) and can be added to the peat surface through
dry and wet atmospheric deposition and fixed from
r 2004 Blackwell Publishing Ltd
N A N D S A C C U M U L AT I O N I N B O G S
atmospheric N2 by microbes (e.g. Hemond, 1983; Urban
& Eisenreich, 1988). S plays an important role in the
oxidation–reduction biogeochemistry of peatlands, influencing the production and emission of methane to
the atmosphere (e.g. Gauci et al., 2002, 2004; Blodau &
Moore, 2003; Vile et al., 2003) and the mineralization of
C (e.g. Wieder & Lang, 1988; Vile et al., 2003). In
addition to a supply from groundwaters, S can be
supplied to peatlands through dry and wet atmospheric
deposition. Based on common C : N and C : S ratios in
peat of 40 : 1 and 250 : 1, northern peatlands contain up
to 10 Pg N and 2 Pg S, stored in the last 10 000 years.
The rates of atmospheric deposition of N and S have
increased in the past 150 years in North America and
Europe, as a result of acid precipitation, and may have
influenced rates of plant production and decomposition
and C cycling. The concentration of N in peat has been
reported in many studies (0.04–0.29%, see Turetsky
et al., 2000) and one can estimate long-term rates of N
accumulation of 0.3–1.0 g N m2 yr1, based on C
accumulation rates of 10–30 g C m2 yr1 (Gorham,
1991) and a C : N ratio of 30 : 1 to 40 : 1. Kuhry & Vitt
(1996) reported long-term N accumulation rates of 0.2–
0.5 g m2 yr1 in peatlands of western Canada. Urban &
Eisenreich (1988) calculated short-term accumulation
rates of about 2 g N m2 yr1 over 20–85 years in a
Minnesota bog, similar to those observed in southern
Sweden (Malmer & Holm, 1984).
Fewer studies report the S concentration in peat or
the rate of S accumulation. Peat S concentrations range
greatly from 10 to 100 mg g1 in a Minnesota bog (Urban
et al., 1989), to 800–7000 mg g1 in peatlands in West
Virginia, Czechoslovakia and Alberta (Wieder & Lang,
1986, 1988; Novák & Wieder, 1992; Turetsky et al., 2000)
and 700–1400 mg g1 in Sphagnum moss samples from
Britain (Bottrell & Novák, 1997). Estimates of S
accumulation in peatlands over 20–100 years range
from 0.2 to 0.5 g m2 yr1 in a Minnesota bog (Urban
et al., 1989), 0.06–0.16 g m2 yr1 in Alberta peatlands
(Turetsky et al., 2000) and 0.3–1.5 g m2 yr1 in a variety
of sites (Novák et al., 1994).
To measure the recent (o150 years) accumulation
rates of N and S and to establish the influence of
atmospheric deposition on these rates, we sampled 23
ombrotrophic bogs along a transect from north-western
Ontario to the Maritimes, Canada, representing a
variation in climate and of atmospheric deposition of
N and S. In each bog, we sampled a hummock and a
hollow and analyzed the upper 50–80 cm for C, N and S
concentration. Using these concentrations and dry bulk
density, we used the 210Pb dating method to estimate
the accumulation rates of these elements over the past
150 years. To estimate retention rates of wet deposition
we carried out a greenhouse experiment on peatland
r 2004 Blackwell Publishing Ltd, Global Change Biology, 11, 356–367
357
cores adding N and S to mesocosms at different loads
and with different water table positions.
In this paper, we:
(i)
(ii)
(iii)
(iv)
(v)
examine the rates of peat accumulation of N and S
over 50, 100 and 150 years and their relationship
with C accumulation over that period;
examine differences in peat hummock and hollow
accumulation of C, N and S at the bogs;
test the hypothesis that rates of N and S accumulation in peat over the past 50 years are correlated
with rates of wet atmospheric deposition of N and
S measured from 1990 to 1996;
estimate rates of atmospheric N and S deposition
over the past 150 years and compare these values
with peat accumulation rates of N and S; and
determine whether N and S added to intact
peatland mesocosms under experimental green
house conditions are retained under different
long- and short-term deposition rates and water
table levels.
Materials and methods
Study area and sampling
Twenty-three ombrotrophic bogs were selected (Table
1) based on the work of Damman & Dowhan (1981),
Glaser & Janssens (1986), Damman (1988), and Gorham
et al. (1985). All peatlands were characterized by an
open canopy of Picea mariana, with Chamaedaphne
calyculata, Ledum groenlandicum, Kalmia angustifolia,
Kalmia polifolia, Vaccinium uliginosum, Vaccinium myrtilloides, Vaccinium oxycoccos, Andromeda polifolia and
scattered Eriophorum spissum. In coastal regions, the
shrub cover was less dominant than in more continental areas but the same species were found in all bogs. A
few Pinus banksiana and Larix laricina were found as a
mixture with Picea mariana in several peatlands of
Ontario and Québec. Hummocks were dominated by
Sphagnum fuscum, with scattered S. magellanicum, S.
capillifolium, Polytrichum spp., Cladina spp., Empetrum
nigrum and Sarracenia purpurea, and hollows by S.
rubellum. and S. angustifolium. In coastal regions, S.
flavicomans was commonly found in the sampled
hummocks as a mixture with S. fuscum. Water table
position during the summer months ranged from 30 to
60 cm beneath the surface in hummocks and 5–20 cm in
hollows.
Recent rates of C, N and S accumulation (RERCA,
RERNA and RERSA, respectively) were measured by
collecting two short cores (50 cm) using a box sampler
(85 85 1000 mm) at a hummock and a hollow site in
each bog. Each short core was double-wrapped in
358 T . M O O R E et al .
polythene bags, and stored at 4 1C. Additional peat
samples were collected down to 80 cm for each core in
case they were needed for further analysis.
Additional peat cores to study the retention process
of added S and N in mesocosms experiments were
taken from hollows in two peatlands at the low and
high end of the sulfur and nitrogen deposition gradient.
The low deposition peatland (o0.4 g N m2 yr1 and
o0.2 g S m2 yr1) was located in the Experimental
Lakes Area (ELA), near Kenora, north-western Ontario;
the mean annual temperature is 1.8 1C and the mean
annual precipitation is 678 mm. It is a small acidic and
oligotrophic peatland located in the north-western
watershed of Lake 239 on the Precambrian Shield
(Bayley et al., 1986). The peatland is dominated by P.
mariana and S. magellanicum, S. angustifolium and S.
fuscum. The high deposition peatland was Mer Bleue,
located east of Ottawa, eastern Ontario (Table 1). It is a
large ombrotrophic bog with a shrub layer of C.
calyculata and K. angustifolia and mosses dominated
by S. magellanicum in hollows (Moore et al., 2002).
Laboratory analyses
Measured volumes of the peat samples were dried to a
constant mass at 70 1C, weighed and the dry bulk
density calculated. Ten samples of 1 cm thickness were
taken from each peat core at 5 cm intervals and a dry
0.5 g subsample from each peat layer was used for
analysis. Peat C and N concentrations were analyzed
using a LECO CHN analyzer (Mississauga, Ontario,
Canada), and S concentration was determined by an
Elementar vario EL analyzer (Hanan, Germany).
210
Pb activity, as estimated by the a emitting 210Po
granddaughter, was determined after spiking with a
209
Po yield tracer. The theory of 210Pb dating (t1/2 5 22.3
Table 1 Location and characteristics of 23 ombrotrophic peatlands sampled in eastern Canada with climatic characteristics and
estimated atmospheric wet deposition of nitrogen (N) and sulfur (S) and N and S deposition zones
Province
Peatland
Nova Scotia Petite Bog
Western Head
Cape Sable
Fourchu
P. E. Island Foxley Moor
New
Point Sapin
Brunswick
Point
Escuminac
Miscou Island
Savoy Bog
Quebec
Lac à la Tortue
Yellow Lake
Mirabel
Despinassy
Bog
Ilets-Jeremie
Bog
Mai Bog
Port Cartier
Bog
Escoumins
Bog
Ontario
Mer Bleue
Baker Bog
Norembego
Nellie Bog
Hislop Bog
Holtrye Bog
Mean
annual
North
West
temperature
(latitude) (longitude) ( 1C)
Mean
N
annual
precipitation N deposition deposition S deposition S deposition
(g m2 yr1) class
(mm)
(g m2 yr1) class
45109 0
43141 0
43128 0
45142 0
46143 0
46159 0
63156 0
65108 0
65136 0
60114 0
64102 0
64151 0
6.5
5.9
5.9
5.5
5.0
4.6
1175
1217
1217
1480
1090
1087
0.34
0.33
0.33
0.29
0.32
0.32
(0.08)
(0.04)
(0.04)
(0.03)
(0.08)
(0.08)
1
1
1
1
1
1
0.41
0.37
0.37
0.35
0.42
0.42
(0.09)
(0.07)
(0.07)
(0.05)
(0.12)
(0.12)
2
1
1
1
2
2
47104 0
64149 0
4.6
1087
0.27 (0.06)
1
0.36 (0.08)
1
47156 0
47147 0
46132 0
48154 0
45141 0
48144 0
64130 0
64136 0
72140 0
71154 0
74103 0
77144 0
4.6
4.6
4.6
1.5
4.8
1.0
1018
1018
1042
835
1030
913
0.28
0.28
0.76
0.32
0.81
0.47
(0.07)
(0.07)
(0.14)
(0.06)
(0.12)
(0.10)
1
1
3
1
3
2
0.38
0.38
0.75
0.42
0.78
0.51
(0.12)
(0.12)
(0.17)
(0.07)
(0.17)
(0.13)
1
1
3
2
3
2
48154 0
68149 0
1.5
996
0.35 (0.06)
1
0.38 (0.09)
1
49157 0
50102 0
67102 0
66156 0
1.0
1.0
1128
1128
0.33 (0.05)
0.29 (0.05)
1
1
0.43 (0.07)
0.41 (0.07)
2
2
48124 0
69121 0
3.0
998
0.36 (0.05)
1
0.42 (0.07)
2
45125 0
49108 0
48159 0
48146 0
48128 0
48132 0
75131 0
90145 0
80142 0
80150 0
80123 0
80148 0
5.8
1.5
0.6
0.9
1.5
0.9
910
763
920
793
876
793
0.81
0.35
0.52
0.52
0.52
0.52
3
1
2
2
2
2
0.82
0.22
0.56
0.56
0.56
0.56
3
1
2
2
2
2
(0.13)
(0.06)
(0.12)
(0.12)
(0.12)
(0.12)
(0.15)
(0.06)
(0.12)
(0.12)
(0.12)
(0.12)
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N A N D S A C C U M U L AT I O N I N B O G S
years) assumes that 210Pb concentrations in peat
decrease exponentially with depth, approaching a low
constant value taken to represent the supported 210Pb
fraction formed within soil as opposed to that deposited from the atmosphere (e.g. Turetsky et al., 2000). The
constant rate of supply model of Appleby & Oldfield
(1978) was applied to calculate the ages of peat layers.
The cumulative dry mass of peat on an aerial basis
(g m2) was calculated as layer thickness weighted
averages from the dry bulk density profile and
converted into C, N and S, based on C, N and S
concentration analyses for surface cores. The accumulated mass of C, N and S above the oldest 210Pb-datable
horizon was divided by the age of this horizon to give
the RERCA, RERNA and RERSA rates (g m2 yr1), for
the 0–50-, 0–100- and 0–150-year periods.
Atmospheric deposition
Rates of atmospheric wet N and S deposition were
calculated using the 7-year mean (1990–1996) results for
eastern North America (R. Vet, C.U. Ro, and D. Ord,
National Atmospheric Chemistry Database and Analysis Facility, Atmospheric Environment Service, Environment Canada, Ontario, SOE Bulletin No. 99-3), using
the station nearest each bog. The bogs were allocated to
three deposition classes: class 1, o0.4; class 2, 0.4–0.6;
and class 3, 40.6 g N or S m2 yr1 (Table 1). The
climate data (Table 1) were derived from the Canadian
Climate Normals (Meteorological Service of Canada,
http://www.msc-smc.ec.gc.ca/climate/climate_normals/index_e.cfm), using the station nearest each bog.
To estimate atmospheric wet N and S deposition over
the 0–50-, 0–100- and 0–150-year time periods, we used
estimates of SO2 and NOx emission for these periods
Table 2
359
from the United States (US Environmental Protection
Agency, 2000) and applied these to the observed 1990–
1996 deposition values.
Mesocosm experiments
To determine the fate of N and S added to peat profiles,
16 peat cores including intact vegetation, 20 cm diameter and 75 cm length, were extracted in PVC tubes
from hollows in late summer to fall and a drainage
mesh and cap attached at the bottom. Pore water
samplers (7 mm outer diameter, 3 mm inner diameter)
were inserted horizontally at 2 cm depth increments.
The water table was initially adjusted with distilled
water to 2–6 cm below the moss surface. After 60 days,
the water table was lowered to about 36 cm in eight of
the cores and constant water tables kept for another 220
days. The temperature was initially 22 1C during the
day and 8 1C at night and after day 50, 12 1C during the
day and 8 1C at night. Relative humidity was maintained at 70% and light intensity was adjusted to
250 mmol m2 s1. Solution was added to the peat cores
with a sprinkler 5–6 days a week and water manually
retrieved at an average outflow of 1.8 mm day1. The
inflowing solute contained H3O 1 , sulfate (26 or
104 mmol L1), nitrate and ammonium (40 or
120 mmol NH4NO3 L1) according to a fractional factorial design with two replicates per treatment (Table 2,
Box et al., 1978). These concentrations were chosen to
represent approximate background (assuming 30% dry
deposition) and three- to fourfold increased deposition
levels than at Mer Bleue. In addition, the inflow
contained dissolved calcium (30 mmol L1), magnesium
sodium
(50 mmol L1),
potassium
(15 mmol L1),
1
(5 mmol L ) and chloride (150–250 mmol L1). Sulfate
Fractional factorial design matrix of the mesocosm experiment and retention of added nitrogen (N) and sulfur (S)
Treatment
Retention at water table (%)
Retention at outflow (%)
Location*
WTw
Nz
S§
NO
3
NH41
SO2
4
NO
3
NH41
SO2
4
MB
MB
MB
MB
ELA
ELA
ELA
ELA
Low
Low
High
High
Low
Low
High
High
High
Low
Low
High
Low
High
High
Low
Low
High
Low
High
Low
High
Low
High
499
97
97
499
96
499
499
498
97
81
82
97
88
90
97
60
0
16
76
89
44
69
77
94
499
498
498
499
498
499
499
498
96
86
85
91
58
85
97
53
81
82
56
97
21
85
21
97
*MB, Mer Bleue; ELA, Experimental Lakes Area 239.
w
Water table (WT) – high 2 cm and low 35 cm below surface.
N additions – low 1.12 and high 3.36 g N m2 yr1 as NH4NO3.
§
S additions – low 0.83 and high 3.33 g m2 yr1 as SO2
4 .
z
r 2004 Blackwell Publishing Ltd, Global Change Biology, 11, 356–367
360 T . M O O R E et al .
0.0
0
0.5
N concentration (%)
1.0
1.5
2.0
2.5
Mer Bleue hummock
Mer Bleue hollow
Hislop hummock
Hislop hollow
Baker hummock
Baker hollow
10
20
30
Depth (cm)
was determined by ion chromatography (Dionex,
Metrosep Anion Dual 1, (Dionex Corporation, Sunnyvale, CA, USA) at 0.5 mL min1 flow rate and chemical
suppression). Dissolved inorganic nitrogen (DIN) was
measured as NH41 and NO
2 1 NO3 using colorimetric
1
methods on a Latchat FIA 8000 series continuous flow
auto-analyzer (Latchat Instruments, Milwaukee, WI,
USA). Nutrient retention was calculated by subtracting
the mass flux of an element at the water table and
outflow, respectively, from the mass flux through
deposition into the mesocosm. The vegetation grew
during the experiment, with estimated rates of photosynthesis equal to or larger than ca. 200 g C m2 yr1
(Blodau et al., 2004).
40
50
60
70
80
Results
90
0.0
0
10
20
0.1
S concentration (%)
0.2
0.3
0.4
0.5
Mer Bleue hummock
Mer Bleue hollow
Hislop hummock
Hislop hollow
Baker hummock
Baker hollow
30
Depth (cm)
Concentrations of peat N ranged from 0.35% to 2.25%
with an average of 0.80% and standard deviation of
0.35%, and S concentrations ranged from 0.07% to
0.55% with an average of 0.18% and standard deviation
of 0.09%. There was a weak but significant overall
correlation between peat N and S concentration (r2 5 0.19,
n 5 466, Po0.001). Ratios of C : N, C : S and N : S
averaged 65 : 1, 318 : 1 and 5 : 1, respectively. In general,
concentrations of N and S increased with depth in the
peat profiles, as illustrated by hummock and hollow
examples from the three deposition classes (Fig. 1), with
a pronounced increase in S concentration in the basal
samples collected from near the water table.
Using N and S concentration and dry bulk density
results and 210Pb dating, the cumulative mass of C, N
and S in the profiles was estimated for the period up to
200 years ago. For each of the three N and S deposition
zones, average values were 11.5, 14.5 and 16.3 kg C m2,
210, 360 and 450 g N m2 and 55, 58 and 110 g S m2; the
accumulation increased with an increase in deposition
by zone (Fig. 2).
Rates of N and S accumulation over the 50-, 100- and
150-year periods averaged 1.86, 1.65 and 1.46 g N
m2 yr1, and 0.38, 0.37 and 0.32 g S m2 yr1, respectively (Fig. 3). There was, however, considerable
variation among sites for the rates for each time period,
with coefficients of variation averaging 47% for N and
43% for S.
There was a strong relationship between C accumulation over the 50-, 100- and 150-year periods with
the accumulation of N (r2 5 0.65–0.68, Po0.001,
Fig. 4). Positive but weaker relationships (r2 5 0.40–
0.44, Po0.001) also existed between C and S accumulation over these periods. In both cases, the position of
the 50-year regression line between C and N or S
accumulation rates was lower than for the other time
periods, indicating a higher C : N or C : S ratio.
40
50
60
70
80
90
Fig. 1 Concentrations of nitrogen (N) and sulfur (S) in
hummock and hollow profiles from sites representing deposition classes 1 (Baker), 2 (Hislop) and 3 (Mer Bleue).
In nearly all cases, the 50-year RERCA rates in
hummocks were larger than those in hollows, the
difference ranging from 40 to 1 75 g C m2 yr1 (Fig.
5). There was a relatively small differentiation in the
RERSA between hummocks and hollows, generally less
than 0.2 g S m2 yr1, with an increase as the difference
in hummock–hollow RERCA increased. Differentiation
between hummock and hollow N accumulation
(RERNA) was larger, ranging from 0.9 to 1 1.5 g N
m2 yr1, with a strong relationship of increasing N
differentiation with increasing C differentiation.
We examined the influence of atmospheric N and S
deposition on RERNA and RERSA by comparing
annual wet deposition rates averaged over the period
r 2004 Blackwell Publishing Ltd, Global Change Biology, 11, 356–367
N A N D S A C C U M U L AT I O N I N B O G S
Fig. 2 Cumulative mass : age relationships for carbon (C),
nitrogen (N) and sulfur (S) and the average relationship for
profiles in the three N or S deposition zones.
1990–1996 for the sites with the 50-year RERNA and
RERSA (Fig. 6). There was a modest but significant
(r2 5 0.30, Po0.001) positive correlation between RERNA and N deposition, and accumulation rates were
about four to five times that of wet deposition. A
weaker, but still significant correlation (r2 5 0.21,
Po0.001) was observed between RERSA and S deposition, with about 75% of the deposited S accumulating in
the peat (Fig. 6). Many profiles showed substantially
lower S accumulation rates than the estimated atmor 2004 Blackwell Publishing Ltd, Global Change Biology, 11, 356–367
361
spheric S deposition rate. Generally, there was a large
variation in RERNA and RERSA at sites exposed to low
rates of atmospheric N and S deposition. The small
number of sites sampled in areas with high rates of
atmospheric N and S deposition limits the strength of
the relationship between deposition and accumulation.
Based on estimated emissions of SO2 and NOx from
the United States (US Environmental Protection
Agency, 2000), we calculated that the overall N and S
wet deposition rates for the 50-, 100- and 150-year
periods would be 83%, 53% and 38% for N and 108%,
92% and 71% for S of the observed 1990–1996 wet
deposition rates. Using these estimated deposition
corrections applied equally to all the sites’ 1990–1996
deposition rates, there was a significant correlation
between deposition and accumulation over the 0–50-,
0–100- and 0–150-year periods, with the best-fit regressions for the longer periods being above those for the
50-year period (Fig. 7).
In the mesocosm experiments, nitrate was nearly
completely retained (497%) above the water table in all
treatments (Table 2) and below the moss canopy,
concentrations were always o3 mmol L1. Ammonium
was retained as well, although to a lesser extent (81–
97%) and below the water table ammonium was on
average released (Table 2). Between 75% and 96% of the
added sulfate was retained by the unsaturated vegetation cover when the water table was close to the peat
surface (Table 2). Sulfate was generally also retained
below the water table (Table 2), probably because of
sulfate reduction, although cases of net release occurred. In treatments with a low water table, the
retention of sulfate by the moss layer was reduced
from 0% to 69% (Table 2). In these treatments, sulfate
concentrations only decreased to ca. 10–30 mmol L1
below the water table (Fig. 8) and as outflow concentrations were similar, the retention was larger in the low
S treatment (Table 2). The significantly negative
correlation between sulfate concentrations at the water
table and rates of photosynthesis determined simultaneously (r2 5 0.53, Po0.05; Blodau et al., 2004) suggested that the uptake of sulfate was related to the
photosynthetic activity of the plant cover. Estimated
average rates of photosynthesis decreased from 48 to
27 mmol m2 day1 under high and low water table
treatments (Blodau et al., 2004).
Discussion
N and S cycling in peatlands is dominated by organic
forms and transformations. Atmospheric deposition of
N is primarily in inorganic forms, ammonium (NH41 )
and nitrate (NO
3 ), which can be rapidly and efficiently
absorbed by the moss layer (Li & Vitt, 1997; Aldous,
362 T . M O O R E et al .
5
1.0
Hummock
Hollow
Hummock mean
Hollow mean
0.8
yr )
3
RERNA (g S m
RERNA (g N m
yr )
4
Hummock
Hollow
Hummock mean
Hollow mean
2
0.6
0.4
0.2
1
50-year
100-year
0
50-year
150-year
100-year
150-year
0.0
Period
Period
Fig. 3 Rates of nitrogen (N) (RERNA) and sulfur (S) accumulation (RERSA) in peat profiles over the 50-, 100- and 150-year periods.
1.0
C−N 50 years
C−N 100 years
C−N 150 years
4
3
100-year: N = 0.029C − 0.79
R = 0.68, P < 0.001
50-, 100- and 150-year peat S accumulation (g m yr )
50-, 100- and 150-year peat N accumulation (g m yr )
5
150-year: N = 0.032C − 0.80
R = 0.67, P < 0.001
2
50-year: N = 0.022C − 0.49
R = 0.65, P < 0.001
1
0
0
50
100
150
50-, 100-and 150- year peat C accumulation (g m
200
yr )
C−S 50 years
C−S 100 years
C−S 150 years
0.8
150-year: S= 0.0044C + 0.010
R = 0.44, P < 0.001
100-year: S = 0.0045C - 0.014
R = 0.41, P < 0.001
0.6
0.4
0.2
50-year: S = 0.0038C - 0.020
R = 0.40, P < 0.001
0.0
0
50
100
150
50-, 100-and 150- year peat C accumulation (g m
200
yr )
Fig. 4 Relationship between rates of nitrogen (N) (RERNA) and sulfur (S) accumulation (RERSA) and C accumulation (RERCA) in peat
profiles over 50-, 100- and 150-year periods.
2002a, b; Nordbakken et al., 2003) and this was
confirmed by the results of our mesocosm experiments.
Bog porewater usually contains small concentrations of
NH41 , almost no NO
3 and the dominant soluble form is
dissolved organic N (DON): at the Mer Bleue bog,
porewater concentrations averaged 0.47, 0.04 and
1.88 mg L1 of NH41 –N, NO
3 –N and DON, respectively (Basiliko & Moore, personal communication).
Peatland plants may be able to take up organic forms of
N (Nordbakken et al., 2003). The absence of porewater
NO
3 and strongly reducing conditions in the upper
part of bog profiles mean that rates of denitrification are
small (e.g. Regina et al., 1996).
Although S inputs from the atmosphere are dom2
inantly in the form of sulfate (SO2
4 ), additions of SO4
to peat porewater result in a rapid disappearance of
SO2
4 (e.g. Bayley et al., 1986) and in bog profiles, most
of the S is found in organic forms (e.g. Wieder & Lang,
1986, 1988; Urban et al., 1989; Novák et al., 1994; Vile
et al., 2003). In ombrotrophic bog profiles, the accumulation of organic matter over the past 100 years occurs
primarily within the acrotelm, the generally oxidized
r 2004 Blackwell Publishing Ltd, Global Change Biology, 11, 356–367
N A N D S A C C U M U L AT I O N I N B O G S
surface peat layer above the water table, so that
reduction–oxidation reactions that are important in
overall S biogeochemistry in wetlands (e.g. Urban et al.,
1989; Mandernack et al., 2000; Blodau et al., 2002) may
play a minor role in the results that we present here,
although there may be anaerobic microsites within the
acrotelm (Vile et al., 2003).
Our results show that significant accumulation of N
and S has occurred over the past 150 years in bogs in
eastern Canada. The range of N accumulation rates is
large (0.5–4.8 g N m2 yr1) over the 50–150-year periods but the average (1.7 g N m2 yr1) is similar to the
2 g N m2 yr1 over 20–85 years in a Minnesota bog
reported by Urban & Eisenreich (1988) and similar to
values reported by Malmer & Holm (1984) in Sweden.
In Norway, Ohlson & Økland (1998) reported a similar
large spatial variation of C (20–100 g C m2 yr1) and N
(0.6–2.1 g N m2 yr1) accumulation in a bog over the
last 125 years. In northern Alberta, Turetsky et al.
(2000) observed N accumulation rates of 0.7 and
0.8 g N m2 yr1 in two bogs. N accumulation rates
were generally larger in hummocks than hollows,
following the pattern of C accumulation.
A fraction of the N accumulation can be explained by
retention of deposited inorganic N. The results of the
mesocosm experiments suggest that during the growing season inorganic N can be fully retained at shortterm deposition rates of up to 4.7 g N m2 yr1. N
saturation, including export of mineral N, has recently
been proposed to occur in peatlands that receive more
than ca. 1.8 g N m2 yr1 in the long term (Lamers et al.,
2000), similar to the largest estimated total deposition
rate along the Canadian transect. This proposal was
based on a simple mass balance consideration of annual
biomass synthesis, N deposition level and C/N ratios
of 40 : 1 to 50 : 1 occurring in Sphagnum and is to some
extent supported by other experimental studies (Williams et al., 1999). At Mer Bleue, at the high deposition
of the Canadian transect, the concentration of N in the
capitulum of S. capillifolium (hummock) and S. magellanicum (hollow) averaged 0.83% and 0.86%, respectively,
in line with observations of N deposition and Sphagnum
N collated by Lamers et al. (2000). Based on studies in
Alberta, Vitt et al. (2003) suggested that the growth of S.
fuscum increased with increasing atmospheric N deposition, reaching a critical total deposition value of
1.4–3.4 g N m2 yr1.
Hu−Ho 50-year N or S accumulation (g m
yr )
2.0
1.5
R
N
S
1.0
N = 0.024C − 0.58
= 0.73, P < 0.001
0.5
0.0
−0.5
−1.0
R
S = 0.004C - 0.15
= 0.42, P < 0.001
−1.5
−2.0
−60
−40 −20
0
20
40
Hu−Ho 50-year C accumulation (g m
60
yr )
80
Fig. 5 The difference between hummock and hollow accumulation of nitrogen (N) and sulfur (S) (RERNA and RERSA) and C
(RERCA) over the past 50 years.
1.0
N
1:1 line
4
50-year peat S accumulation (g m−2 yr −1)
5
50-year peat N accumulation (g m−2 yr −1)
363
y = 2.84x + 0.67
R = 0.32, P < 0.001
3
2
1
S
1:1 line
0.8
0.6
0.4
R
y = 0.66x + 0.07
= 0.28, P < 0.001
0.2
0.0
0
0.0
0.2
0.4
0.6
0.8
Wet atmospheric N deposition, 1990−1996 (g
1.0
m−2
yr−1)
0.0
0.2
0.4
0.6
0.8
Wet atmospheric S deposition, 1990−1996 (g
1.0
m−2
yr−1)
Fig. 6 Relationship between nitrogen (N) accumulation (RERNA) and sulfur (S) accumulation (RERSA) in peat profiles over 50 years
and atmospheric wet deposition of N and S between 1990 and 1996.
r 2004 Blackwell Publishing Ltd, Global Change Biology, 11, 356–367
364 T . M O O R E et al .
5
100-year: y = 0.58x + 0.12
R
0.8
Peat RERNA (g S m
100-year: y = 4.84x + 0.58
R = 0.28, P = 0.001
3
2
50-year: y = 3.50x + 0.64
R = 0.29, P < 0.001
1
= 0.23, P < 0.05
150-year: y = 0.68x + 0.10
R = 0.30, P < 0.001
yr )
yr )
4
Peat RERNA (g N m
1.0
50-year
100-year
150-year
1:1 line
150-year: y = 6.30x + 0.46
R = 0.32, P < 0.001
0.6
0.4
50-year: y = 0.68x + 0.03
R = 0.33, P < 0.001
50-year
100-year
150-year
1:1 line
0.2
0
0.0
0
0.2
0.4
0.6
0.8
1
Estimated wet atmospheric N deposition (g m yr )
0
0.2
0.4
0.6
0.8
1
Estimated wet atmospheric S deposition (g m yr )
Fig. 7 Relationship between nitrogen (N) accumulation (RERNA) and sulfur (S) accumulation (RERSA) in peat profiles over 50-,
100- and 150-year periods and estimated atmospheric wet deposition of N and S over those periods.
Concentration (µmol L−1)
50
100
150 0
Concentration (µmol L−1)
20
40
60
0
0
10
10
Water table
20
Depth (cm)
80
20
30
30
40
40
50
50
60
60
70
Depth (cm)
0
MB
ELA
70
80
80
, high Wt, high S;
, low Wt, high S
, high Wt, low S;
, low Wt, low S
Fig. 8 Pore water concentrations of SO2
in the mesocosm
4
treatments for the Mer Bleue (MB) (left) and Experimental Lakes
Area (ELA) (right) peat cores. Corrected for the evapotranspiration from the mesocosms, inflow concentrations were about 75
and 220 mmol L1 NH4NO3 and 45 and 190 mmol L1 SO2
4 . Bars
indicate standard deviations.
On average, N accumulation in the bog cores of this
study is about four times that in wet atmospheric
deposition. Other sources of N therefore have to be
considered. These include organic N deposition, dry
deposition and N2 fixation. DON forms a variable
proportion of atmospheric N deposition and may
increase the overall deposition by about 50% above
the wet deposition (Neff et al., 2002; Cornell et al., 2003).
Dry deposition is difficult to measure and estimate and
may be smaller in bog systems than forests, because of
the small leaf surface area: at Mer Bleue, for example,
the vascular leaf area index is 1.3 (Moore et al., 2002).
Thus, contemporary total N atmospheric deposition
may be 75–100% larger than wet deposition, amounting
to 0.4–1.6 g N m2 yr1 at these bog sites in eastern
Canada. This range is close to the critical N deposition
value of about 1.5 g m2 yr1 for Sphagnum growth and
N retention cited by Lamers et al. (2000) and Vitt et al.
(2003).
Annual N2 fixation estimates in ombrotrophic bogs
range from 0.1 and 1.0 g N m2 yr1 in Minnesota and
Massachusetts, respectively (Chapman & Hemond,
1982; Hemond, 1983; Urban & Eisenreich, 1988).
Measurements have been infrequent and sparse and
rates may be underestimated, as has been recently
found in boreal forests (DeLuca et al., 2002).
Losses of N from the peat may arise from denitrification, although rates are small (o0.2 g N m2 yr1) in
acidic bogs with a generally low water table (Hemond,
1983; Urban & Eisenreich, 1988; Regina et al., 1996).
Aquatic export of N occurs as NH41 , NO
3 , DON and
particulate N. Losses of NH41 may account for 0.1–
0.2 g N m2 yr1 and NO
3 losses are small, probably
o0.05 g N m2 yr1 (Hemond, 1983; Urban & Eisenreich, 1988). Export of DON may be larger, between 0.1
and 0.3 g N m2 yr1, based on DOC export of 5–
10 g C m2 yr1 and DOC : DON ratios of 30 : 1 to 50 : 1
(Hemond, 1983; Urban & Eisenreich, 1988; Fraser et al.,
2001).
Thus, the bogs along our transect receive 0.4–1.6 g N
m2 yr1 in precipitation and 0.1–1.0 g N m2 yr1 in N2
fixation, and lose through denitrification (o0.2 g N
m2 yr1) and aquatic export (0.2–0.5 g N m2 yr1).
These net gains, ranging from 0 to 2.3 g N m2 yr1,
r 2004 Blackwell Publishing Ltd, Global Change Biology, 11, 356–367
N A N D S A C C U M U L AT I O N I N B O G S
are similar to the N accumulation rates over the 50–150year periods, which average 1.5–2.0 g N m2 yr1. There
was a correlation between increasing atmospheric N
deposition and N accumulation rates, although there is
great variability among sites with similar N deposition.
In part, this variation may be related to the hummock–
hollow microtopography: hummocks had larger C and
N accumulation rates than hollows along our transect
(Fig. 5). In a Norwegian bog, Ohlson & Økland (1988)
recorded coefficients of variation of 56% and 57% for
recent C and N accumulation rates in peat samples
taken within a 20 m 20 m area. This variability may be
associated with higher rates of plant production and N
fixation in hummocks, whereas atmospheric N deposition rates are unlikely to be very different between
hummocks and hollows. Leaf biomass in bog hummocks averaged 156 g m2, while the average for
hollows was 106 g m2, although Sphagnum capitulum
mass was similar at 176 and 172 g m2 (Turunen et al.,
2004). In Alberta, Vitt et al. (2003) noted that production
of S. fuscum increased with increased atmospheric N
deposition, within the range encountered in eastern
Canada, without a major increase in N concentration in
the underlying peat. Although there are changes in
climate, particularly mean annual temperature (Table 1)
and growing season, along the transect, it appears that
the primary cause of increased N accumulation in the
peat cores is increased atmospheric N deposition.
The range of S accumulation rates in the eastern
Canadian bogs (0.1–0.9 g S m2 yr1) encompassed that
found in other wetlands noted earlier (Urban et al.,
1989; Novák et al., 1994; Turetsky et al., 2000). Our
results suggest that, on average, the bogs retained about
75–90% of the recent wet atmospheric deposition of S in
the acrotelm. Inputs of dissolved organic S (DOS) are
probably insignificant (Park et al., 2003), but dry
deposition of SO2
4 may increase total S deposition by
up to 50%. There can be substantial S losses (0.1–
0.3 g m2 yr1) from wetland catchments, as both SO2
4
and DOS (Devito et al., 1989; Urban et al., 1989; Devito,
1995; Gorham et al., 1998). Gaseous losses of H2S are
probably negligible since concentrations in the pore
water of bogs seem to remain in the lower micromolar
range (Steinmann & Shotyk, 1997; Blodau et al., 2002).
The retention rate observed in these eastern Canadian
bogs is similar to that noted by Hemond (1983) at
Thoreau’s bog (77%) and by Urban et al. (1989) at bogs
in Minnesota and north-western Ontario (58% and 91%,
respectively).
Our estimates of S retention rates were also in
agreement with the results of the mesocosm experiment, which showed similar retention efficiencies with
the exception of combined low water tables and high
2
deposition. In contrast to NO
3 , SO4 concentrations in
r 2004 Blackwell Publishing Ltd, Global Change Biology, 11, 356–367
365
the pore water did not decrease below 10–30 mmol L1.
This concentration may be because of either the release
of sulfates from the peat matrix (Mandernack et al.,
2000) or kinetic limits to microbial utilization of SO24
and was responsible for the lower S retention rates in
the mesocosm experiments. The results of both transect
and mesocosm approach, nevertheless, suggest that
North American bogs are sinks for S. This sink function
may be lost under more severe S pollution in parts of
Europe and the Appalachian mountain range, where
SO2
4 has been found in peat porewater at concentrations of 50–300 mmol L1 (Wieder & Lang, 1988; Wieder
et al., 1990; Nedwell & Watson, 1995; Watson &
Nedwell, 1998). Large continuous SO24 export in
streams was also reported from a central European
peatland (Hruska et al., 1996). The retention capacity for
S has also been partially lost in North American
swamps affected by acidic precipitation and large input
rates of N and S from upland ecosystems (LaZerte,
1993).
The maintenance of the stoichiometric relationship
between C and N in these bogs (Fig. 4) suggests that
elevated rates of atmospheric deposition of N have led
to increased rates of C accumulation. Turunen et al.
(2004) observed a significant relationship (partial
regression, r2 5 0.28 and 0.38 for hummocks and
hollows, Fig. 3) between wet atmospheric N deposition
during 1990–1996 and 50-year C accumulation, along
this Canadian bog transect. As noted above, this may
arise from N being the major nutrient limiting plant
productivity in boreal bogs, with increased Sphagnum
production associated with N deposition rates encountered here (e.g. Malmer, 1990; Aerts et al., 1992;
Berendse et al., 2001; Vitt et al., 2003). The increased C
accumulation rate with elevated N deposition may also
arise from a slowing in the rate of organic matter
decomposition, as observed in European forests by
Berg & Matzner (1997) and slower heterotrophic
respiration in the uppermost 5 cm of the peat profile
(Williams & Silcock, 1997).
Conclusions
Ombrotrophic bogs in eastern Canada have accumulated N and S over the past 150 years at rates of 0.5–
4.8 g N m2 yr1 and 0.1–0.9 g S m2 yr1. There was a
high degree of spatial variability in accumulation rates,
particularly between hummocks and hollows. The N
and S accumulation rates were correlated with recent
rates of atmospheric N and S deposition over the range
0.3–0.8 g N m2 yr1 and 0.2–1.0 g S m2 yr1. The ratio
between accumulation and wet atmospheric deposition
averaged about 4 : 1 and 0.75 : 1 for N and S, respectively. The results confirm that in eastern Canadian
366 T . M O O R E et al .
bogs, the plant canopy and upper peat control the
subsurface dynamics of S and N through their filter
function, converting mineral N and S into organic
forms.
Acknowledgements
Funding for this project was received from the Canadian
Climate Change Action Fund (CCAF), the Natural Sciences
and Engineering Research Council of Canada and the FCARsupported Centre for Climate and Global Change Research.
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